Method for data transmission in a local area network

ABSTRACT

A method for data transmission in a local area network, wherein data is transmitted on a medium access control layer within successive time frames between a plurality of first nodes comprising client nodes and a second node within reach of the first nodes, and a coordinator node for the first nodes where a time frame comprises a plurality of time slots. Each time slot is assigned to a first node that is a slot owner node being exclusively allowed to start transmitting data at a time within a first interval at the beginning of the time slot. At least one first nodes of the plurality of first nodes is allowed to use the time slot based on a contention based access to transmit the data in a second interval succeeding the first interval where the slot owner node has not started transmitting data at a time within the first interval.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/EP2010/050493 filed18 Jan. 2010. Priority is claimed on EP Application No. 09000659 filed19 Jan. 2009, and EP Application No. 09010309 filed 10 Aug. 2009, thecontents of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention related to data communications networks and, moreparticularly to a method for data transmission in a local area networkand to a corresponding network.

In the following, the term “local area network” refers to any type ofnetwork restricted to a local area, such as wireless local area network(WLAN) networks or personal area networks, e.g. according to theInstitute of Electrical and Electronics Engineers (IEEE) standard802.15.4.

For many applications, local area networks have to fulfil certainrequirements with respect to a data transmission in the network. Forexample, in wireless factory automation sensor systems, in which thelocal area network comprises sensor nodes and a base station collectingdata from the sensor nodes, cyclic data traffic characteristics have tobe kept with respect to low latency and packet loss rates. To fulfilthese requirements, certain mechanisms for a data transmission areprovided in the L2 or Media Access (MAC) layer of the well-known OpenSystems Interconnection (OSI) reference model. In local area networks,nodes often seek to transmit data at the same time, which may result indata collisions. Hence, mechanisms are provided for avoiding suchcollisions.

In IEEE 802: Part 15.4: Wireless Medium Access Control (MAC) andPhysical Layer (PHY) Specifications for Low-Rate Wireless Personal AreaNetworks (WPANs) IEEE Std 802.15.4™, September 2006 referring tolow-rate wireless personal area networks, a contention based datatransmission using the Carrier Sense Multiple Access/Collision Avoidance(CSMA/CA) method is described. Due to this method, a time slot for datatransmission is not assigned to a specific node, but each node in thenetwork may use the wireless medium for data transmission. In order toavoid collisions, a node intending to send a data packet determinesusing a clear channel assessment whether the radio interface is free,i.e., whether another node is currently transmitting on the radiointerface. In cases where no other data transmissions are detected bythe node, it will start to transmit its data. In order to lower the riskof collisions with other nodes trying to send at the same time, thesending node waits a random delay before starting the clear channelassessment.

In the CSMA/CA method described in IEEE 802: Part 15.4: Wireless MediumAccess Control (MAC) and Physical Layer (PHY) Specifications forLow-Rate Wireless Personal Area Networks (WPANs) IEEE Std 802.15.4™,September 2006, the so-called hidden node problem may occur. Thisproblem is a situation in which two nodes in the network, which are outof reach with each other, intend to send data based on the CSMA/CAmethod to the same receiver. However, both nodes cannot hear each other.As a result, they will determine that the radio interface is free andwill start sending data. This results in a data collision at thereceiver. In order to recognize the effects of the hidden node problem,IEEE 802: Part 15.4: Wireless Medium Access Control (MAC) and PhysicalLayer (PHY) Specifications for Low-Rate Wireless Personal Area Networks(WPANs) IEEE Std 802.15.4™, September 2006 describes a mechanism bywhich a node transmitting data requests a positive acknowledgement fromthe receiver. If such an acknowledgement is not sent due to a collisioncaused by the hidden node problem, the data transmission of the nodewill be repeated.

In IEEE 802: Part 11: Wireless LAN Medium Access Control (MAC) andPhysical Layer (PHY) Specifications IEEE Std 802.11™, June 2007referring to the WLAN IEEE standard 802.11, a modified CSMA/CA method isdescribed that includes a virtual carrier sense mechanism to avoid thehidden node problem.

According to this mechanism, a node wishing to transmit data reservesthe radio interface for a predetermined time interval by sending arequest-to-send packet to the receiver, where the packet includes thelength of the time interval to be reserved. The receiver answers therequest-to-send packet by sending a clear-to-send packet, which alsoincludes the reserved time length, back to the sending node. Therequest-to-send and clear-to-send packets are broadcast in the networkand all other nodes receiving those packets regard the radio interfaceas occupied for the time length specified in the packets. Particularlydue to the transmission of the clear-to-send packets, nodes are informedabout a forthcoming data transmission which is in reach of the receiverbut not in their reach. IEEE 802: Part 11: Wireless LAN Medium AccessControl (MAC) and Physical Layer (PHY) Specifications IEEE Std 802.11™,June 2007 also describes a clear to send-to-self mechanism where asending node addresses a clear-to-send packet to itself. All neighboringnodes receiving this packet will refrain from transmitting data withinthe time length specified in the packet.

The above described hidden node problem will not only occur incontention based transmission methods but also in TDMA basedtransmission methods Time Division Multiple Access (TDMA). In TDMA basedmethods, each time slot in a corresponding time frame is reserved for aspecific node being the slot owner node which can exclusively send inthis time slot. In some TDMA based systems, a time slot may also be usedby other nodes than the slot owner node in case that a data transmissioncannot be detected by the other nodes after the expiry of a certaininterval within the time slot. Such a TDMA based system is described inMichael Bahr, Norbert Vicari, Ludwig Winkel: Shared Group Timeslots IEEE802.15-Dokument 15-08/0827r0, November 2008 referring to the proposalfor standard IEEE 802.15.4e being an extension of standard IEEE 802.15.4defined for a sensor network usable in factory automation environments.In the aforementioned TDMA systems the situation may occur in which anode that is not within reach of the slot owner node starts a datatransmission that is in parallel to the slot owner node. This situationis to be avoided because a TDMA system guarantees to a certain extentthat a slot owner node can transmit data in its time slot without anydisturbances from other nodes.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method for datatransmission in a local area network by which a reliable time division(TDMA) based transmission combined with contention based access isachieved.

This and other objects and advantages are achieved by a local areanetwork and method of the in which data is transmitted on the mediumaccess control (MAC) layer within successive time frames between aplurality of first nodes comprising client nodes and a second nodewithin reach of the first nodes and comprising a coordinator node forthe first nodes. A respective time frame comprises a plurality of timeslots and each time slot is assigned to a first node that is a slotowner node which is exclusively allowed to start transmitting data at atime within a first interval at the beginning of the time slot. Thismechanism allows a time division multiple access (TDMA) based access forthe slot owner node of the time slot.

The method of the invention also enables a contention based access fortransmitting data. The term “contention based access” is to beinterpreted broadly and refers to any method providing mechanismsscheduling a data transmission where different nodes can try to use thesame time slot for transmitting data. In a preferred embodiment, theabove mentioned CSMA/CA method is used as the contention based access.

To implement the contention based access, one or more first nodes of theplurality of first nodes, e.g., all first nodes or a subset of them, areallowed to use the time slot of a slot owner node using the contentionbased access for transmitting data in a second interval succeeding thefirst interval in cases in which a slot owner node has not startedtransmitting data within the first interval.

If the second node determines at the end of the first interval that theslot owner node has not started transmitting data, it will broadcast apermission message in the local are network. According to the invention,each first node is allowed to use the contention based access fortransmitting data only after receiving the permission message from thesecond node.

The invention is based on using the coordinating second node in thenetwork, which is within reach of all first nodes, to initiate acontention based data transmission by sending a corresponding permissionmessage. Hence, all first nodes are informed explicitly about the factthat the slot owner node does not transmit data. Therefore, a hiddennode that is not within reach of the slot owner node cannot starttransmitting data parallel to a data transmission of the slot ownernode. As a consequence, data collisions occurring at the second node areavoided, thus enhancing the reliability of the data transmission for theslot owner node.

In a preferred embodiment of the invention, the data is transmitted in awireless personal area network, i.e., in a wireless sensor network,where each first node represents a sensor transmitting sensor data tothe second node representing a base station in the sensor network. Anexample of such a network is described in Michael Bahr, Norbert Vicari,Ludwig Winkel: Proposal for Factory Automation, IEEE 802.15-Dokument15-08/0572r0, August 2008. In the following, a sensor refers to awireless device having the function to send data. If such a device alsohas the function of receiving data from the base station, it may also becalled actuator. In a preferred embodiment, the data in the wirelesspersonal area network is transmitted according to the aforementionedIEEE standard 802.15.4, i.e., in accordance with the standard proposalIEEE 802.15.4e provided for sensor and actuator networks for factoryautomation which is reflected in Michael Bahr, Norbert Vicari, LudwigWinkel: Proposal for Factory Automation IEEE 802.15-Dokument15-08/0572r0, August 2008.

The permission message sent by the second node in accordance with themethod of the invention preferably includes at least an identificationof its message type. This enables the first nodes to identify themessage as a permission message. In another embodiment, the permissionmessage further includes an identification of the second node and/or ofthe local area network to which the second node belongs. This enablesthe use of the method in a system of overlapping local area networks,where some of the first nodes belong to several local area networks withdifferent second nodes. By including the identification of the networkor the corresponding second node, different local area networks can bedistinguished from each other.

In another preferred embodiment of the invention, the permission messageis kept very short. This is possible because the permission messageneeds not include a reserved time length, as in the case in theabove-described clear-to-send messages of the WLAN standard.

In another preferred embodiment of the invention, a second hidden nodeproblem is solved. This second hidden node problem refers to a collisionthat may occur during the contention based access within the second timeinterval when two nodes intending to send data via the contention basedaccess are out of reach from each other. To do so, each first nodeintending to use the time slot based on the contention based access fortransmitting data at first broadcasts a request message in the localarea network within the second time interval. Upon the first receipt ofa request message at the second node, an admission message identifyingthe first node from which the request message originates is broadcast bythe second node in the local area network. As a consequence, each firstnode receiving the admission message and not being identified in theadmission message refrains from a data transmission, whereas the firstnode receiving the admission message and being identified in theadmission message starts transmitting its data. Hence, the admissionmessage sent by the first node ensures that only one node startstransmitting data even in cases in which the nodes intending to transmitdata are not within reach of each other.

In a preferred embodiment, the above defined request message includes atleast an identification of its message type and an identification of thefirst node that has sent the request message. As a result, the secondnode is able to determine the identification of the first node to beincluded in the admission message.

In order to ensure that the request messages sent by several first nodesdo not collide, those messages are preferably sent based on a contentionbased access, i.e., by a CSMA/CA method.

In another embodiment of the invention, the admission message at leastincludes an identification of its message type and an identification ofthe first node from which the corresponding request message originates.As a result, a first node is able to determine whether it is allowed totransmit data by the contention based access.

In another embodiment of the method according to the invention, therequest message and/or the admission message further include anidentification of the second node and/or of the local area network towhich the second node belongs. Due to this feature, the method of theinvention may be used in a system of overlapping local area networks.

The request and/or admission messages can be very short. Particularly,those messages need not include a reserved time length, as in the casefor the above described request-to-send packets and clear-to-sendpackets of the WLAN standard.

In another embodiment of the invention, the permission message and/orthe request message and/or the admission message have a common commandframe format, particularly the command frame format defined in the IEEEstandard 801.15.4e. An example of such a command frame format isdescribed in Michael Bahr, Norbert Vicari, Ludwig Winkel: Proposal forFactory Automation IEEE 802.15-Dokument 15-08/0572r0, August 2008.

In another embodiment of the invention, data transmissions from thefirst nodes to the second node are acknowledged by the second node. In apreferred embodiment, the second node sends an acknowledgement afterreceiving data from the slot owner node. This acknowledgement ispreferably sent in a synchronizing or beacon slot at the beginning ofthe next time frame. Furthermore, the second node may send anacknowledgement after receiving data using the above describedcontention based access. This acknowledgement is preferably sent withinthe time slot in which the data is received by the contention basedaccess.

In another embodiment of the invention, the time slot includes, inaddition to the above-described first and second time intervals, a thirdtime interval succeeding the second time interval that is used to sendnetwork announcements by the second node if no data is transmittedwithin the first and second time intervals.

Besides the above described method, the invention also provides a localarea network comprising a plurality of first nodes comprising clientnodes and a second node within reach of the first nodes and comprising acoordinator node for the first nodes, where the network is configuredsuch that the method for data transmission in accordance with the methodof the invention can be performed.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention. It should be furtherunderstood that the drawings are not necessarily drawn to scale andthat, unless otherwise indicated, they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in detail withrespect to the accompanying drawings, in which:

FIG. 1 is an exemplary illustration of a personal area network in whichan embodiment of the method in accordance with the invention isimplemented;

FIG. 2 is an illustration of a structure of successive time frames usedfor transmitting data in accordance with an embodiment of the invention;

FIG. 3 to FIG. 5 illustrate specific formats for messages beingtransmitted in accordance with an embodiment of the invention;

FIG. 6 to FIG. 8 are diagrams illustrating different scenarios fortransmitting data in accordance with an embodiment of the invention; and

FIG. 9 is a flow chart illustrating the method in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, an embodiment of the invention is described based on adata transmission according to the proposal to the Institute ofElectrical and Electronics Engineers (IEEE) standard 802.15.4e. Ingeneral, the IEEE standard 802.15.4 defines the medium access control(MAC) layer according to the Open Systems Interconnection (OSI)reference model for wireless and low power transmission of sensor data.The proposal to the IEEE standard 802.15.4e is a specific amendment ofIEEE standard 802.15.4 used for data transmission between sensors andactuators in factory automation. A sensor refers to a wireless devicedesigned for transmitting data to a base station or gateway. A devicehaving the functionality of a sensor and which is additionally able tohandle data transmissions from the base station to the device is calledactuator. Hence, an actuator can be regarded as a sensor having theadditional functionality of a downlink transmission from the gateway tothe actuator. In accordance with the invention, a sensor or actuatorcorresponds to a first node, whereas the gateway or base stationcorresponds to a second node.

As previously explained, the IEEE standard 802.15.4e is designed forfactory automation, where sensors and actuators are located, forexample, at robots, suspension tracks and portable tools in theautomotive industry and collect data on machine tools, such as millingor turning machines, and control revolving robots. Further applicationareas are control of conveyor belts in cargo or logistics scenarios orspecial engineering machines. Depending on the specific needs ondifferent factory automation branches, many more examples can be named.Common to sensor applications in factory automation is the requirementof low latency and high cyclic determinism. As a consequence, theperformance should allow for reading sensor data from 20 sensors within10 milliseconds. The IEEE standard 802.15.4e fulfils the needs offactory automation by using a fine granular Time Division MultipleAccess (TDMA) access, where in a superframe structure guaranteed timeslots for deterministic access are assigned to corresponding firstnodes.

FIG. 1 shows an example of a wireless sensor network in a star topologyof the IEEE standard 802.15.4e. The network comprises four first nodesA, B, C and D, i.e., corresponding sensors or actuators, communicatingwith a second node GW comprising a gateway to the wireless network.Based on the star topology shown in FIG. 1, where all data transmissionsare directed from a first node to the second node or all datatransmissions are directed from the second node to first node.

In accordance with the method of the invention, data is transmitted fromthe corresponding first nodes A to D to the coordinating second node GWin successive time frames. Those time frames are illustrated in FIG. 2.With specific reference to FIG. 2, shown therein along the horizontaltime axis t are two successive time frames, where each time frame refersto a superframe SF and comprises a synchronizing time slot BS at thebeginning and a plurality of transmission slots TA, TB, TC and TD usedfor data transmissions following the synchronizing time slot BS. Thesynchronizing time slot BS includes a beacon B that is used forsynchronizing the data transmission between the first nodes A to D andthe second node GW. Moreover, the beacon B includes acknowledgementsindicating whether TDMA based data transmissions in the transmissionframe of a preceding superframe have been successful.

Each of the time slots TA to TD forms a shared group time slot in whichseveral smaller time slot units of equal length are concatenated. Inaccordance with the invention, a shared group time slot corresponds to atime slot. In the example of FIG. 2, each of the time slots TA to TD isassigned to a fixed first node (also called slot owner node in thefollowing) in the network of FIG. 1. Particularly, time slot TA isassigned to first node A, time slot TB is assigned to first node B, timeslot TC is assigned to first node C and time slot TD is assigned tofirst node D. Based on this assignment, a TDMA based method is used fortransmitting data by a corresponding first node in the time slotassigned to this first node.

The method as described herein combines a TDMA based data transmissionwith a contention based data transmission using a Carrier Sense MultipleAccess/Collision Avoidance (CSMA/CA) method. The CSMA/CA method for datatransmission is used in a time slot in cases in which the time slot isnot used by the corresponding slot owner node. To do so, each of thetime slots TA to TD is divided in further sub-intervals. The structureof these sub-intervals is shown in FIG. 2 for the time slot TB. Thisstructure is the same for all other shared group time slots within thetransmission frame TF.

As shown in FIG. 2, a shared group time slot extends from a start timet0 to an end time t3. Here, the interval between those times is dividedinto three fixed time intervals I1, I2 and I3. The time interval I1extends from time t0 to time t1, the time interval I2 extends from timet1 to time t2 and the time interval I3 extends from time t2 to time t3.In accordance with the subsequently described embodiment, the use of ashared group time slot is to a certain extend guaranteed for the slotowner node. Particularly, the slot owner node has the exclusive right tostart a data transmission within the interval I1 of its time slot. Onlyif the slot owner node does not start a data transmission within thistime interval, is a CSMA/CA method then used for data transmissionwithin the succeeding interval I2 of the shared group time slot.According to the CSMA/CA method, each of the first nodes A to D can tryto send data within the time interval I2. If no data is transmittedwithin the time interval I2 by any of the first nodes A to D, then thegateway GW uses the succeeding time interval I3 to announce the networkby corresponding announcement messages.

The method as-described corresponds to the transmission method describedin Michael Bahr, Norbert Vicari, Ludwig Winkel: Shared Group TimeslotsIEEE 802.15-Dokument 15-08/0827r0, November 2008. The inventiondescribed subsequently provides improvements of the conventional methodand solves the hidden node problem that may occur when using the methodof Michael Bahr, Norbert Vicari, Ludwig Winkel: Shared Group TimeslotsIEEE 802.15-Dokument 15-08/0827r0, November 2008. This hidden nodeproblem is herein illustrated based on FIG. 1. In the network of FIG. 1,a star topology ensures that the gateway GW is within reach of eachfirst node A, B, C and D. Nevertheless, there is no guarantee that eachfirst node is within reach of all the other first nodes in the network.In accordance with the situation shown in FIG. 1, sensor nodes A and Bare within reach of each other and sensor nodes C and D are within reachof each other. However, sensor nodes A and B are not within reach ofsensor nodes C and D. Hence, first nodes C and D are hidden nodes forfirst nodes A and B. Therefore, in case in which first node A or Btransmits data in its corresponding time slot assigned to it, wherefirst nodes C and D will not detect those data transmissions. Hence,first nodes C and D may send data within interval I2 of time slot TA orTB based on a CSMA/CA method in parallel to a transmission by first nodeA or B in the corresponding time slots A or B because both first nodes Cand D will detect a clear channel. Consequently, unacceptable datacollisions will occur because there is a certain guarantee for the firstnodes A and B to be able to transmit data within the corresponding timeslots TA and TB.

In accordance with the method of the invention, the above describedhidden node problem is solved by broadcasting a clear-to-send (CTS)packet (by the second node GW. This packet is designated in FIG. 3 asCTS-1 and comprises an embodiment of a permission message. Particularly,in cases in which the second node GW detects, after the expiry of theinterval I1 that the corresponding slot owner node has not started adata transmission, the second node GW will broadcast the CTS packet toall first nodes in the network. The format of the packet CTS-1 isillustrated in FIG. 3 and is based on the command frame structure asdefined in Michael Bahr, Norbert Vicari, Ludwig Winkel: Proposal forFactory Automation IEEE 802.15-Dokument 15-08/0572r0, August 2008.According to FIG. 3, the first line of the format indicates the lengthof the corresponding sub-units of the packet CTS-1 in octets, i.e., inbytes. The CTS packet comprises a MAC header MHR, a MAC payload MP and aMAC footer MFR. The MAC header has a length of 1 byte and includes ashortened frame control field SFC specifying a frame type. The structureof this shortened frame control field is described in Michael Bahr,Norbert Vicari, Ludwig Winkel: Proposal for Factory Automation IEEE802.15-Dokument 15-08/0572r0, August 2008. In the formats of FIG. 3 toFIG. 5, the shortened frame control field SFC indicates that the frameis a command frame. The MAC header MHR is followed by the MAC payload MPcomprising two fields of one byte length. The first field specifies thatthe command frame type FT is a first type of CTS packet. The secondfield includes a network ID NID which specifies the network in which theCTS message CTS-1 is transmitted.

Instead of a network identification, the second field may specify theidentification of the corresponding gateway GW, e.g., the sendingaddress of the gateway. The use of the network ID or the gatewayidentification is optional and only relevant in cases in which there aremore than one overlapping networks of the type shown in FIG. 1 todistinguish the messages sent in the different networks. The CTS messageCTS-1 includes a final field having the length of two bytes. This fieldis the MAC footer and includes a frame check sequence FCS correspondingto a checksum for the packet. The structure of the MAC footer is alsodescribed in the aforementioned Michael Bahr, Norbert Vicari, LudwigWinkel: Proposal for Factory Automation IEEE 802.15-Dokument15-08/0572r0, August 2008 document.

In accordance with the disclosed embodiments of the invention asdescribed herein, the trigger for allowing a contention based datatransmission is not the expiry of time interval I1 but the receipt ofthe above described packet CTS-1. As a consequence, the above describedhidden node problem is solved because the gateway GW is within reach ofall first nodes so that it is ensured that data is not transmitted bythe slot owner node when the CTS packet CTS-1 is broadcast by the secondnode. The packet CTS-1 according to FIG. 3 does not indicate a timeinterval, contrary to CTS packets sent in the WLAN standard. This isbecause the packet CTS-1 does not have the function of reserving theradio interface in the network for a predetermined time. Rather, thepacket informs all first nodes in the network that the nodes are allowedto send data based on a CSMA/CA method. All nodes in the network knowthe length of the interval I2 from the network configuration. Hence, thenodes know which frame they can transmit using a contention based accesswithin the interval I2.

In the above-described scenario, another second hidden node problem mayoccur during the contention based access within the time interval I2.This is because a node wishing to send data packets within the timeinterval I2 listens to the radio interface to check if the interface isfree, i.e., whether it is possible to send data. Hence, in the networkillustrated in FIG. 1, a data collision will occur when first node A orB and also first node C or D intend to send data using a contentionbased access because nodes A and B are not within reach of nodes C andD. This second hidden node problem is not as severe as the abovedescribed hidden node problem because the contention based access doesnot give a guarantee for the nodes to transmit data, contrary to theTDMA based scheme as previously described in which each first node ownsa certain time slot reserved for this node.

In an alternative embodiment, the second hidden node problem is alsosolved. Here, after all first nodes have received the above describedpacket CTS-1 from the second node GW, those first nodes wishing to senddata on a contention based access within the time interval I2 initiallytransmit request-to-send packet RTS to the gateway GW. This RTS packetforms an embodiment of a request message. The transmission of the packetRTS is performed on a CSMA/CA method, i.e., those nodes wishing to sendthis packet will delay the transmission by a random time. This mechanismensures that almost always only one RTS packet reaches the gateway GW.

The format of the request-to-send packet RTS is shown in FIG. 4. As withpacket CTS-1, this format is based on the command frame format describedin Michael Bahr, Norbert Vicari, Ludwig Winkel: Proposal for FactoryAutomation IEEE 802.15-Dokument 15-08/0572r0, August 2008.Correspondingly with FIG. 3, the first line in the packet RTS indicatesthe length of the respective fields, where the number refers to thelength in bytes. The packet in FIG. 4 has a similar structure as thepacket in FIG. 3. Particularly, the packet in FIG. 4 includes a MACheader MHR comprising a shortened frame control field SFC, a MAC payloadMP and a MAC footer FTR comprising a frame check sequence FCS of twobyte length. The MAC payload MP of the packet RTS includes three fieldseach having one byte length, whereas the MAC payload of the packet CTS-1only includes two bytes. The MAC payload of the packet RTS includes afirst field which indicates that the frame type FT is an RTS packet.Additionally, the packet RTS includes an identification of the firstnode which has sent this packet. This is indicated by a short originatoraddress SOA in a second field of the MAC payload MP. In addition, anetwork identification NID is included in the MAC payload of the packetRTS. This NID specifies the network to which the first node sending thepacket belongs. Instead of the network identification, the networkaddress of the gateway GW may be included in the last field of the MACpayload. As with the packet CTS-1, the network identification isoptional and only relevant when several overlapping networks exist. Atime interval needs not be indicated in the packet RTS.

After the gateway GW has received the above described packet RTS, thegateway GW will immediately broadcast a second type of clear-to-sendpacket CTS-2, the structure of which is shown in FIG. 5. This CTS-2packet comprises an embodiment of an admission message. The format ofthe packet CTS-2 is similar to the format of the packet RTS shown inFIG. 4. As before, the format of the packet CTS-2 is based on thecommand frame format defined in Michael Bahr, Norbert Vicari, LudwigWinkel Proposal for Factory Automation IEEE 802.15-Dokument15-08/0572r0, August 2008. Correspondingly with the packets of FIG. 3and FIG. 4, the first line in the packet format of FIG. 5 indicates thebyte length of the corresponding fields of the packet. The packet CTS-2includes a MAC header MHR comprising a shortened frame control fieldSFC, a MAC payload MP and a MAC footer MFR including a frame checksequence FCS. The MAC payload MP includes in a first field an indicationthat the command frame type FT is a CTS-2 packet. In a second field ofthe MAC payload MP, the address of the first node from which the formerpacket RTS was received by the second node is included as a shortdestination address SDA. Furthermore, the identification NID of thenetwork is included in the last field of the MAC payload MP. Instead ofthis network identification, an identification of the gateway GW may beincluded in this field. As before, the field of the networkidentification is optional and only relevant in cases in which severaloverlapping networks exist. A time interval need not be included in thepacket CTS-2.

The packet CTS-2 is received by all first nodes in the network. Thosefirst nodes having another address as the short destination address SDAindicated in the packet CTS-2 will refrain from transmitting data withinthe time interval I2. Contrary to that, the first node having the shortdestination address indicated in the packet CTS-2 will starttransmitting data in the time interval I2. The gateway GW is withinreach for all first nodes. As a result, the packet CTS-2 will bereceived by all first nodes. Hence, it is ensured that the first nodespecified in the packet CTS-2 will receive the packet. Furthermore, thisnode will be the only node which uses the time interval I2 for a datatransmission. Consequently, collisions are avoided and the second hiddennode problem as specified above is solved.

In the following, the method of the invention will be described based ondifferent scenarios for data transmission in the network as shown inFIG. 1. For simplicity, it is assumed that first node D is switched offin the network, i.e., this first node D does not participate in the datatransmission. Each of FIGS. 6 to 8 shows a shared group time slot TAthat is owned by the first node A. Each of the FIGS. 6 to 8 indicatesthe behaviour of the gateway GW and the behaviour of the first nodes Ato C in different transmission scenarios. As already explained withrespect to FIG. 1, first nodes A and B are within reach of each other,whereas sensor node C is out of reach of sensor nodes A and B. In FIGS.6 to 8, time intervals in which a node listens to the radio interfaceand does not transmit data are indicated by dotted bars having referencenumeral LI (LI=listen). In contrast, the transmission of messagesperformed by a node are indicated by hatched bars having referencenumeral FT (FT=frame transmission). The status of the nodes in whitesections of the bars in FIGS. 6 to 8 might be listen or sleep and arenot relevant in the following description. Hence, the status in thesesections is not indicated.

FIG. 6 shows a scenario where first node A transmits data within thetime slot TA owned by first node A, and first node C would like to usetime slot TA for a data transmission. The data transmission of firstnode A starts within the time interval I1 and is indicated by the barFT. Within time slot TA, the gateway GW will constantly listen to theradio interface but it will not send any messages. Furthermore, firstnode C, which cannot hear the frame transmission of node A, will alsoconstantly listen to the radio interface without detecting any frametransmission. First node C does not receive a CTS packet CTS-1. As aresult, first node C will not start its data transmission in intervalI2. Therefore, the data transmission by first node A will not bedisturbed.

FIG. 7 shows a scenario where first node A does not start transmittingdata within the time interval I1 and first node C would like to use timeslot TA for a data transmission. Gateway GW detects that first node Ahas not started transmitting data within time interval I1. Gateway GWwill broadcast the above described CTS packet CTS-1. This packet isreceived by all first nodes A to C that can start a contention basedaccess within the time slot 12 after having received the packet CTS-1.FIG. 7 thus illustrates an embodiment where a contention based accessusing a CSMA/CA method is used without transmitting the above describedpackets RTS and CTS-2. Hence, in the presently contemplated embodiment,collisions may occur between first nodes not in reach to each other andwishing to send data. In the scenario of FIG. 7, only first node Cintends to send data based on a CSMA/CA method within the time intervalI2. The random delay occurring in the CSMA/CA method until node C startsframe transmission FT is indicated by reference numeral TD in FIG. 7.After this random delay TD, the frame transmission FT occurs withoutcollisions because neither first node A nor first node B wish to senddata using a contention based method. FIG. 7 shows an acknowledged datatransmission where, after the transmission of the frame to gateway GW, acorresponding acknowledgment ACK is transmitted by gateway GW.

FIG. 8 shows a scenario implementing a method in accordance with thedisclosed embodiments of the invention where the above describedmessages RTS and CTS-2 are transmitted. In FIG. 8, first node A does notstart transmitting data within the time interval I1 and first nodes Band C seek to use time slot TA for a data transmission. As aconsequence, the packet CTS-1 is initially broadcast by the gateway GW.Upon receipt of the packet CTS-1, all first nodes A to C may use thetime slot TA for contention based access. In the scenario of FIG. 8,both first nodes B and C wish to send data within the time interval I2.To do so, a CSMA/CA method is used for sending the packet RTS by nodes Band C. In the scenario of FIG. 8, the randomly selected time for sendingthe packet RTS is such that node B sends the packet RTS first. Thispacket is received by gateway GW, which answers by a corresponding CTSpacket CTS-2 including the address of first node B. Both first nodes Band C receive packet CTS-2. First node B will recognize that the packetCTS-2 includes its address as the destination address SDA and will startframe transmission FT thereafter. The total delay induced by the CSMA/CAmethod until frame transmission is indicated in FIG. 8 by referencenumeral TD. In contrast, node C determines that its own address isdifferent from the destination address of packet CTS-2. As a result,first node C will not start a frame transmission FT. Analogously to thescenario of FIG. 7, the frame transmission FT is acknowledged by gatewayGW with a corresponding acknowledgement ACK.

The embodiments of the invention described in the foregoing solve thehidden node problem which results in collisions of data transmissionsfrom first nodes that are not within reach of each other for a TDMAbased access, and also for a contention based access within personalarea networks, i.e., based on the IEEE standard 802.15.4e. As the hiddennode problem no longer occurs in the above described embodiments, thereliability of data transmissions in a network is improved.

FIG. 9 is a flow chart of a method for data transmission in a local areanetwork, where the data is transmitted on a medium access control layerwithin successive time frames between a plurality of first nodescomprising client nodes and a second node within reach of the pluralityof first nodes and a coordinator node for the plurality of first nodes,and a time frame comprising a plurality of time slots. The methodcomprises assigning each of the plurality of time slots to a first nodeof the plurality of the first nodes which is a slot owner node beingexclusively allowed to start transmitting data at a time within a firstinterval at a beginning of a time slot of the plurality of time slots,as indicated in step 910.

One or more of the first nodes of the plurality of first nodes ispermitted to use the time slot of the plurality of time slots using acontention based access to transmit the data in a second intervalsucceeding the first interval in cases in which a slot owner node hasnot started transmitting data at a time within the first interval, asindicated in step 920.

At an end of the first interval the second node determines whether theslot owner node has started transmitting data, as indicated in step 930.The second node broadcasts a permission message in the local areanetwork, if the second node determines that the slot owner node has notstarted transmitting the data, as indicated in step 940.

Each first node of the plurality of nodes is permitted to use the timeslot of the plurality of times slot using the contention based access totransmit the date only after receiving the permission message from thesecond node, as indicated in step 950.

Thus, while there are shown, described and pointed out fundamental novelfeatures of the invention as applied to preferred embodiments thereof,it will be understood that various omissions and substitutions andchanges in the form and details of the illustrated method and apparatus,and in their operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention.

Moreover, it should be recognized that methods and structures shownand/or described in connection with any disclosed form or embodiment ofthe invention may be incorporated in any other disclosed or described orsuggested form or embodiment as a general matter of design choice.

The invention claimed is:
 1. A method for data transmission in a localarea network, the data being transmitted on a medium access controllayer within successive time frames between a plurality of first nodescomprising client nodes and a second node within reach of the pluralityof first nodes and a coordinator node for the plurality of first nodes,and a time frame comprising a plurality of time slots, the methodcomprising: assigning each of the plurality of time slots to a firstnode of the plurality of the first nodes which is a slot owner nodebeing exclusively allowed to start transmitting data at a time within afirst interval at a beginning of a time slot of the plurality of timeslots; permitting one or more first nodes of the plurality of firstnodes to use the time slot of the plurality of time slots using acontention based access to transmit the data in a second intervalsucceeding the first interval in cases in which a slot owner node hasnot started transmitting data at a time within the first interval;determining by the second node, at an end of the first interval whetherthe slot owner node has started transmitting data; broadcasting, by thesecond node, a permission message in the local area network, if thesecond node determines that the slot owner node has not startedtransmitting the data; and permitting each first node of the pluralityof nodes to use the time slot of the plurality of times slot using thecontention based access to transmit the date only after receiving thepermission message from the second node.
 2. The method according toclaim 1, wherein the data is transmitted in a wireless personal areanetwork, each first node of the plurality of first nodes representing asensor transmitting sensor data to the second node which represents abase station in the sensor network.
 3. The method according to claim 2,wherein the data in the wireless personal area network is transmitted inaccordance with IEEE standard 802.15.4e.
 4. The method according toclaim 2, wherein the wireless personal area network comprises a wirelesssensor network.
 5. The method according to claim 1, wherein thecontention based access is based on a Carrier Sense MultipleAccess/Collision Avoidance method.
 6. The method according to claim 1,wherein the permission message includes at least an identification ofits message type.
 7. The method of claim 6, wherein the permissionmessage further includes an identification of at least one of the secondnode and the local area network to which the second node belongs.
 8. Themethod according to claim 7, wherein the permission message does notinclude a reserved time length.
 9. The method according to claim 6,wherein the permission message does not include a reserved time length.10. The method according to claim 1, further comprising: sending,initially from each first node of the plurality of first nodes intendingto use the time slot of the plurality of times slot using the contentionbased access to transmit the data, a request message in the local areanetwork within the second interval; broadcasting, by the second node inthe local area network, an admission message identifying the first nodeof the plurality of first nodes from which the request messageoriginates upon a first receipt of a request message at the second nodewithin the second interval; wherein each first node of the plurality offirst nodes the admission message and not being identified in theadmission message refrains from a data transmission; and wherein thefirst node of the plurality of first nodes receiving the admissionmessage and being identified in the admission message startstransmitting its data.
 11. The method according to claim 10, wherein therequest message at least includes an identification of its message typeand an identification of the first node of the plurality of first nodeswhich has sent the request message.
 12. The method according to claim10, wherein the request message is sent using a contention based access.13. The method according to claim 12, wherein the contention basedaccess is a Carrier Sense Multiple Access/Collision Avoidance method.14. The method according to claim 10, wherein the admission messageincludes at least an identification of its message type and anidentification of the first node of the plurality of first nodes fromwhich the corresponding request message originates.
 15. The methodaccording to claim 10, wherein at least one of the request message andthe admission message further includes an identification of at least oneof the second node and the local area network to which the second nodebelongs.
 16. The method according to claim 10, wherein at least one ofthe request message and the admission message does not include areserved time length.
 17. The method according to claim 10, wherein atleast one of the permission message, the request message and theadmission message have a common command frame format, particularly thecommand frame format defined in IEEE standard 802.15.4e.
 18. The methodaccording to 10, wherein the common command frame format is the commandframe format defined in IEEE standard 802.15.4e.
 19. The methodaccording to claim 1, wherein the second node sends an acknowledgementafter receiving data from the slot owner node, the acknowledgement beingsent in a synchronizing slot at a beginning of a next time frame. 20.The method according to claim 1, wherein the second node sends anacknowledgement after receiving data using the contention based access,the acknowledgement being sent within the time slot of the plurality oftime slots in which the data is received using the contention basedaccess.
 21. The method according to claim 1, wherein the time slot ofthe plurality of time slots includes a third time interval succeedingthe second time interval being used to send network announcements by thesecond node f no data is transmitted within the first and secondintervals.
 22. A local area network, comprising: a plurality of firstnodes comprising client nodes and a second node within reach of theplurality of first nodes; and a coordinator node for the plurality offirst nodes; wherein the network is configured for data transmission ona medium access control layer within successive time frames between theplurality of first nodes and the second node, a time frame comprising aplurality of time slots; wherein each time slot of the plurality of timeslots is assigned to a first node of the plurality of first nodes whichis a slot owner node being exclusively allowed to start transmittingdata at a time within a first interval at the beginning of the time slotof the plurality of time slots; wherein at least one first node of theplurality of first nodes is permitted to use the time slot based on acontention based access to transmit the data in a second intervalsucceeding the first interval in cases in which the slot owner node hasnot started to transmit the data at a time within the first interval;wherein at an end of the first interval the second node determineswhether the slot owner node has started to transmit the data; whereinthe second node broadcasts a permission message in the local areanetwork, if it determines that the slot owner node has not startedtransmitting data; and wherein each first node of the plurality of firstnodes is allowed to use the time slot of the plurality of time slotsbased on the contention based access to transmit the data only afterreceiving the permission message from the second node.
 23. The networkaccording to claim 22, wherein the network is configured to transmit thedata in a wireless personal area network, each first node of theplurality of first nodes representing a sensor transmitting sensor datato the second node which represents a base station in the sensornetwork.
 24. A node implemented in a method for data transmission in alocal area network, wherein data is transmitted on a Media AccessControl (MAC) layer within successive time frames between a plurality offirst nodes comprising client nodes, and the node including a processorand memory and being within reach of the plurality of first nodes andcomprising a coordinator node for the plurality of first nodes, a timeframe comprising a plurality of time slots, wherein each time slot isassigned to a first node of the plurality of first nodes which is a slotowner node being exclusively allowed to start transmitting data at atime within a first interval at a beginning of a time slot of theplurality of time slots; wherein at least one first node of theplurality of first nodes are allowed to use the time slot via acontention based access for transmitting data in a second intervalsucceeding the first interval during cases in which the slot owner nodehas not started transmitting data at the time within the first interval;wherein the processor causes the node to determine whether the slotowner node has started transmitting data at the end of the firstinterval; and wherein the processor causes the node to broadcast apermission message in the local area network, if the slot owner node hasnot started transmitting data, the permission message allowing eachfirst node of the plurality of first nodes to use the time slot via acontention based access for transmitting data only after receiving thepermission message from the node.